1. Field of the Invention
The present invention relates to a thin film magnetic memory device, and particularly to a thin film magnetic memory device provided with memory cells having MTJs (Magnetic Tunnel Junctions)
2. Description of the Background Art
Attention is being given to an MRAM (Magnetic Random Access Memory) device as a memory device, which can nonvolatilely store data with low power consumption. The MRAM device is a memory device, in which a plurality of thin film magnetic members are formed in a semiconductor integrated circuit for nonvolatilely storing data, and random access to each thin film magnetic member is allowed.
Particularly, in recent years, it has been announced that a performance of the MRAM device can be dramatically improved by using the thin film magnetic members, which utilize the MTJs (magnetic tunnel junctions), as memory cells. The MRAM device with memory cells having the magnetic tunnel junctions has been disclosed in technical references such as “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”, ISSCC Digest of Technical Papers, TA7.2, February 2000, “Nonvolatile RAM based on Magnetic Tunnel Junction Elements”, ISSCC Digest of Technical Papers, TA7.3, February 2000, and “A 256 kb 3.0V 1T1MTJ Nonvolatile Magnetoresistive RAM”, ISSCC Digest of Technical Papers, TA7.6, February 2001.
FIG. 21 conceptually shows a structure of a memory cell, which has a magnetic tunnel junction, and may be merely referred to as an “MTJ memory cell” hereinafter.
Referring to FIG. 21, an MTJ memory cell includes a tunneling magneto-resistance element TMR having an electric resistance, which is variable in accordance with a level of storage data, and an access element ATR for forming a path of a data read current Is passing through tunneling magneto-resistance element TMR in a data read operation. Access element ATR is typically formed of a field-effect transistor, and therefore may be referred to as an “access transistor ATR” hereinafter. Access transistor ATR is coupled between tunneling magneto-resistance element TMR and a fixed voltage (ground voltage GND).
For the MTJ memory cell, the structure includes a write word line WWL for instructing data writing, a read word line RWL for executing data reading and a bit line BL, which is a data line for transmitting an electric signal in accordance with the data level of the storage data.
FIG. 22 conceptually shows an operation of reading data from the MTJ memory cell.
Referring to FIG. 22, tunneling magneto-resistance element TMR has a ferromagnetic material layer, which has a fixed and uniform magnetization direction, and may be merely referred to as a “fixed magnetic layer” hereinafter, and a ferromagnetic material layer VL, which is magnetized in a direction depending on an externally applied magnetic field, and may be merely referred to as a “free magnetic layer” hereinafter. A tunneling barrier (tunneling film) TB formed of an insulator film is disposed between fixed magnetic layer FL and free magnetic layer VL. Free magnetic layer VL is magnetized in the same direction as fixed magnetic layer FL or in the opposite direction in accordance with the level of the storage data to be written. Fixed magnetic layer FL, tunneling barrier TB and free magnetic layer VL form a magnetic tunnel junction.
In the data read operation, access transistor ATR is turned on in response to activation of read word line RWL. Thereby, data read current Is can flow through a current path formed of bit line BL, tunneling magneto-resistance element TMR, access transistor ATR and ground voltage GND.
Tunneling magneto-resistance element TMR has an electric resistance, which is variable depending on a correlation in magnetization direction between fixed magnetic layer FL and free magnetic layer VL. More specifically, when the fixed magnetic layer FL and free magnetic layer VL are magnetized in the same (parallel) direction, the electric resistance of tunneling magneto-resistance element TMR is smaller than that in the case where these layers FL and VL are magnetized in the opposite directions (parallel opposite directions), respectively.
Accordingly, by magnetizing free magnetic layer VL in a direction depending on the storage data, the voltage change caused in tunneling magneto-resistance element TMR by data read current Is changes depending on the storage data level. For example, if data read current Is is passed through tunneling magneto-resistance element TMR after precharging bit line BL to a predetermined voltage, the storage data of the MTJ memory cell can be read out by detecting the voltage on bit line BL.
FIG. 23 conceptually shows an operation of writing data in the MTJ memory cell.
Referring to FIG. 23, read word line RWL is inactive, and access transistor ATR is turned off in the data write operation. In this state, the data write currents for magnetizing free magnetic layer VL in the direction depending on the level of the write data are supplied to write word line WWL and bit line BL, respectively. The magnetization direction of free magnetic layer VL depends on the respective data write currents flowing through write word line WWL and bit line BL.
FIG. 24 conceptually shows a relationship between the data write current and the magnetization direction of the tunneling magneto-resistance element in the data write operation for the MTJ memory cell.
Referring to FIG. 24, an abscissa H(EA) gives a magnetic field, which is applied in an easy axis (EA) to free magnetic layer VL of tunneling magneto-resistance element TMR. An ordinate H(HA) indicates a magnetic field acting in a hard axis (HA) on free magnetic layer VL. Magnetic fields H(EA) and H(HA) correspond to two magnetic fields produced by currents flowing through bit line BL and write word line WWL, respectively.
In the MTJ memory cell, the fixed magnetization direction of fixed magnetic layer FL is parallel to the easy axis of free magnetic layer VL, and free magnetic layer VL is magnetized in the magnetization easy direction, and particularly in the same parallel direction, which is the same direction as fixed magnetic layer FL, or in the opposite parallel direction, which is opposite to the above direction, depending on the level (“1” or “0”) of the storage data. In the following description, the electric resistances of tunneling magneto-resistance element TMR, which correspond to the two magnetization directions of free magnetic layer VL, are indicated by R1 and R0 (R1>R0), respectively. The MTJ memory cell can selectively store data (“1” and “0”) of one bit corresponding to the two magnetization directions of free magnetic layer VL.
The magnetization direction of free magnetic layer VL can be rewritten only when a sum of applied magnetic fields H(EA) and H(HA) falls within a region outside an asteroid characteristic line shown in FIG. 18. Therefore, the magnetization direction of free magnetic layer VL does not switch when the data write magnetic fields applied thereto have intensities corresponding to a region inside the asteroid characteristic line.
As can be seen from the asteroid characteristic line, the magnetization threshold required for changing the magnetization direction along the magnetization easy shaft can be lowered by applying the magnetic field in the direction of the hard axis to free magnetic layer VL.
When the operation point in the data write operation is designed, for example, as shown in FIG. 24, the data write magnetic field in the MTJ cell selected as a data write target is designed such that the data write magnetic field in the direction of the easy axis has an intensity of HWR. Thus, the data write current flowing through bit line BL or write word line WWL is designed to take a value, which can provide the data write magnetic field of HWR. In general, data write magnetic field HWR is represented by a sum of a switching magnetic field HSW required for switching the magnetization direction and a margin ΔH. Thus, it is represented by an expression of HWR=HSW+ΔH.
For rewriting the storage data of the MTJ memory cell, i.e., the magnetization direction of tunneling magneto-resistance element TMR, it is necessary to pass the data write currents at a predetermined level or higher through write word line WWL and bit line BL. Thereby, free magnetic layer VL in tunneling magneto-resistance element TMR is magnetized in the same parallel direction as fixed magnetic layer FL or opposite parallel direction in accordance with the direction of the data write magnetic field along the easy axis (EA). The magnetization direction, which was once written into tunneling magneto-resistance element TMR, and thus the storage data of MTJ memory cell is held nonvolatilely until next data writing is executed.
As described above, the electric resistance of tunneling magneto-resistance element TMR is variable in accordance with the magnetization direction, which is rewritable by the data write magnetic field applied thereto. Therefore, nonvolatile data storage can be executed by establishing a correlation between two magnetization directions of free magnetic layer VL in tunneling magneto-resistance element TMR and levels (“1” and “0”) of the storage data.
In the data write operation of the MRAM device, as described above, it is necessary to switch the magnetization direction of tunneling magneto-resistance element TMR in the MTJ memory cell, which is selected as a data write target. Therefore, it is necessary to control the directions of the data write currents flowing through write word line WWL and bit line BL in accordance with the level of the write data. This complicates a structure of a circuit system supplying the data write current, and increases a chip size of the MRAM device.